Heat treatment devices have been used to form diffusion layers or form silicon oxide or nitride films on semiconductor or glass substrates in the manufacture of electronic devices. These substrates are typically thin wafers made of silicon or other semiconductor materials. The description of the device hereinafter will be provided in reference to wafer substrates, it being understood that the apparatus is equally suitable for treating any thin glass or semiconductor sheets, and treatment of any or all of these materials are considered to be within the scope of this invention.
These devices provide the desired heat treatment by heating the wafers in a reactor or heating chamber while introducing inert or reactive gases into the chamber. These heating chambers are surrounded by heating elements enclosed within an insulated shell. In order to treat large numbers of wafers in a single heat treatment operation, it is conventional to support the wafers, one above the other in a parallel orientation, in a wafer boat. This combination is referred to hereinafter as a wafer stack.
Vertical furnaces generally have the furnace and coaxial wafer boat aligned along a vertical axis. The wafer boat loaded with wafers to be treated is raised into the furnace through a bottom opening before the treatment cycle and lowered from the furnace after treatment. A preferred vertical furnace designed to reduce particulate contaminants by eliminating gas eddy areas in the reaction chamber is described in U.S. Pat. No. 5,320,680.
The prior art furnaces are limited by the ability of their heating systems to rapidly raise and lower furnace temperatures. Japanese patent application publication Hei 4-184923, to Nishimura et al. (Jul. 1, 1992), describes a heat treatment apparatus designed to reduce the heating time. The heating and cooling cycle times for the Nishimura et al. system are limited by the provision of additional heat sink materials to support the wafers, since they increase the power requirements and times for the heating and cooling phases.
Commonly assigned, copending Application Ser. No. 08/827,542 filed Mar. 28, 1997 describes a rapid heating and cooling furnace with wafer boat structures which protect wafers during rapid heating and cooling cycles. This furnace provides a cooling cycle in which air is drawn through the furnace between the outer reaction chamber wall and the heating elements to rapidly remove heat and lower the temperature of the furnace, including the reaction chamber and the wafers. The heated air is removed through the system exhaust. An optional heat exchanger in the exhaust duct is described as an option to lower the exhaust gas temperature and protect thermally vulnerable seals or gaskets in the exhaust system. This system requires the facility in which the furnace will be operated to an adequate supply of cooling gases and an exhaust system suitable for removing the heated air.